1543267 - Abspos flex children in a row-reverse/column-reverse/wrap-reverse container need to be aligned to end (flex-start) side

Status: RESOLVED FIXED

(Core :: Layout: Flexbox, defect, P3)

Description

[Daniel Holbert [:dholbert]]

Attachment: testcase 1

Right now we fail a few subtests in these WPT tests:
https://wpt.fyi/results/css/css-flexbox/position-absolute-001.html
https://wpt.fyi/results/css/css-flexbox/position-absolute-002.html

The issues are all for cases where there's a flex container with a reversed main axis or cross axis (from flex-direction: row-reverse or column-reverse, or flex-wrap: wrap-reverse).

The test asserts that the static position of an abspos child should be at the "end" side of the flex container (the flex-start side, since the relevant flex axis is reversed with a *-reverse keyword in these cases).

We incorrectly place the abspos child at the start side. But we're wrong -- the relevant behavior here is the "normal" behavior (since "normal" is the default align/justify keyword), and the spec says:

normal Behavior
For replaced absolutely-positioned boxes,
the 'normal' keyword behaves as 'start'.

For all other absolutely-positioned boxes, 'normal' behaves as 'stretch'
https://drafts.csswg.org/css-align/#align-abspos

The last line is the relevant one here -- normal is supposed to behave as stretch, and stretch "falls back to flex-start generally", which in this flex-axis-reversed scenario would mean the far/"end" side of the flex container.

Testcase attached -- currently we do the right thing for flex-start (the lime rect) but not for stretch or for the default behavior (the red and orange rects).

Comment 1

[David Burns :automatedtester]

css/css-flexbox/position-absolute-002.html

Comment 2

[Daniel Holbert [:dholbert]]

The WPT tests have moved to a subdirectory:
https://wpt.fyi/results/css/css-flexbox/abspos/position-absolute-001.html
https://wpt.fyi/results/css/css-flexbox/abspos/position-absolute-002.html

Comment 3

[Ting-Yu Lin [:TYLin] (PDT, UTC-7)]

Attachment: Bug 1543267 Part 1 - Make 'stretch' fallback to 'flex-start' for flexbox's abspos children. r?dholbert

In Flexbox spec 4.1, Example 3 [1]:
... since the absolutely-positioned box is considered to be
"fixed-size", a value of stretch is treated the same as flex-start.

Also, per Alignment 3 spec [2]:
The default fallback alignment for 'stretch' is 'flex-start'.

Thus, when computing the alignment for flexbox's abspos children in
CSSAlignmentForAbsPosChild(), we convert 'stretch' to 'flex-start', and let the
subsequent logic convert 'flex-start' to either 'start' or 'end', because
nsAbsoluteContainingBlock don't know how to deal with the flex-relative axis.

This patch makes us behave the same as Google Chrome on the modified testcases.

[1] https://drafts.csswg.org/css-flexbox/#abspos-items
[2] https://drafts.csswg.org/css-align/#valdef-align-content-stretch

Comment 4

[Ting-Yu Lin [:TYLin] (PDT, UTC-7)]

Attachment: Bug 1543267 Part 2 - Make 'space-between' fallback to 'flex-start' for flexbox's abspos children. r?dholbert

Per Alignment spec [1], the default fallback alignment for 'space-between' is
'flex-start'.

[1] https://drafts.csswg.org/css-align/#valdef-align-content-space-between

Depends on D134543

Comment 6

[Daniel Holbert [:dholbert]]

Thanks for taking this and for updating the test expectations!

Making sense of things: it looks like the default fallback value changed in the spec, in the time since this behavior was originally implemented. For space-between, it changed here:
https://github.com/w3c/csswg-drafts/commit/36ca6ddf33fa4f2934c663a8c74eab0d59beaa69#diff-273c5b2266723c0e21f50eff93d04ae10a9a0ce8cc67825e68a64c74f4056073

...and I'm not about the history for stretch (dug around a little bit but didn't immediately see where it had previously been specified as falling back to start).

Comment 7

[Daniel Holbert [:dholbert]]

Aha -- for stretch it looks like the specced fallback behavior changed here (the old spec text said Otherwise, this is equivalent to ''start''):
https://github.com/w3c/csswg-drafts/commit/1e436030985a2adbc9b54b5aade1ff882c5ed85d#diff-0b7c7fab23fb5f18a5f7ae5a9ec43fc8e9e4f1ac5f8ee16a02cb7d1ae2360e95L218

Comment 8

[Ting-Yu Lin [:TYLin] (PDT, UTC-7)]

Thanks for digging into the spec history!

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/ce9e6a8b981c
Part 1 - Make 'stretch' fallback to 'flex-start' for flexbox's abspos children. r=dholbert
https://hg.mozilla.org/integration/autoland/rev/791f9fca748e
Part 2 - Make 'space-between' fallback to 'flex-start' for flexbox's abspos children. r=dholbert

Comment 10

[Web Platform Test Sync Bot (Matrix: #interop:mozilla.org)]

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/32179 for changes under testing/web-platform/tests

Comment 11

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/ce9e6a8b981c
https://hg.mozilla.org/mozilla-central/rev/791f9fca748e

Comment 12

[Web Platform Test Sync Bot (Matrix: #interop:mozilla.org)]

Upstream PR merged by moz-wptsync-bot